Process for loading a fibrous slurry with filler

ABSTRACT

A process to load a fibrous slurry with a filler, in particular with calcium carbonate (fiber loading). The loaded fibrous slurry is divided into at least two fractions, a light fraction and a heavy fraction, for which purpose hydrocyclones are used in one embodiment. In this case, the different fractions have a different level of loading, that is to say the quantity of precipitated fillers adhering to the fibers is different in each case. There is the possibility of feeding the less highly loaded fibers back into the loading process, in order to increase the level of loading of this fraction further.

This is a continuation of PCT application No. PCT/EP2007/002535, entitled “METHOD OF LOADING A PULP SUSPENSION WITH FILLER”, filed Mar. 22, 2007, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for loading a fibrous slurry with a filler.

2. Description of the Related Art

A number of processes from fiber loading technology for loading pulp fibers with calcium carbonate are already known. In U.S. Pat. No. 5,223,090 a process is described in which fibrous material with elongated fibers having a cell wall surrounding a cavity is used, the fibers having a moisture content which is sufficient to form a dewatered suspension of a pulp. In this case, the fibers have a moisture content which corresponds to a proportion of 40 to 50% of the weight of the fibers. The water is present substantially in the interior of the fibers and within the fiber walls. Alternatively, calcium oxide or calcium hydroxide is then added to the pulp, so that at least some of the calcium oxide or calcium hydroxide introduced will associate with the water present in the pulp. The fibrous material is then brought into connection with carbon dioxide, and simultaneously subjected to a shear mixing process, in order to produce a fiber material having a considerable quantity of calcium carbonate in the hollow interiors and within the fiber walls of the pulp fibers as a result of precipitation.

DE 102 04 254 A1 discloses a further process for loading a fibrous slurry. The process comprises the following steps: introduction of calcium hydroxide in liquid or dry form or of calcium oxide into the fibrous slurry, introduction of gaseous carbon dioxide into the fibrous slurry, and precipitation of calcium carbonate by the carbon dioxide.

What is needed in the art is a process for loading a fibrous slurry in which the advantages of the loading process are increased, and possibly a considerably higher loading effect can also be achieved.

SUMMARY OF THE INVENTION

The present invention provides a process, for producing fibrous material loaded with calcium carbonate (FLPCC or fiber loaded precipitated calcium carbonate), in particular for pulp production or for the use of pulp in papermaking. The raw fibrous material to be loaded is produced, for example, from recycled paper, from DIP (deinked paper), from secondary fibrous material, bleached or unbleached chemical pulp, groundwood of any type, any paper raw chemical pulp, bleached or unbleached sulphate pulp, finished material broke, linen, cotton and/or hemp fibers (predominantly used for cigarette paper) and/or any other raw paper material which is used in a paper machine. The process can be employed irrespective of the raw fibrous material to be loaded.

The area of application of the present invention extends to the production of paper and pulp and the process technology including the filler produced, and comprises areas of application in all grades of paper including packaging paper grades and the broke accumulating during production, which has a filler content of between 1% and 60% and/or has a white top layer with a filler content of between 1% and 60%. The filler content can preferably lie between 5% and 50%.

If a fibrous slurry is treated with fiber loading technology during the paper production, this results in a completely novel product, which has new and improved properties as compared to products known on the market. Such a process makes it possible to precipitate filler (calcium carbonate) directly during stock preparation in a paper mill, the filler being distributed uniformly and deposited exclusively on and in the fibrous material, in particular the paper fibers.

In the process according to the present invention, the fractionation can beneficially be carried out with hydrocyclones, which produce a high centrifugal field which, for example, corresponds to 200 times the effect of the earth's force of gravity.

Another possibility lies in the application of pressure screens, in particular if they are designed as fiber fractionators, that is to say are provided with fine screen openings. Of course, multi-stage separation systems and/or combinations of various separation principles are also conceivable.

The process according to the present invention is carried out advantageously such that the different effect of the loading of the fibrous material suspension is either utilized or, by means of appropriate measures, is partly or completely compensated again. In the loading process on which this invention is based, for example, a fibrous material suspension composed of various types of fibers is formed. On the basis of the different properties, it is possible that the loading is not the same in all the participating fibers. In addition, a statistical influence is conceivable such that, on the basis of parameter fluctuations during the precipitation process, be they temporal or local, the precipitation is not uniform. The different level of loading, that is to say the quantity of precipitated crystals adhering to the fibers, permits fractionation following the loading process, in which at least two fractions with a different level of loading are formed. Since it is to be assumed that more highly loaded fibers are heavier than less highly loaded ones, the designations heavy fraction for fibers with a higher level of loading and light fraction for fibers with a lower level of loading are chosen here.

Depending on the requirements on the process, two possibilities are conceivable in order to treat the different fractions further.

One embodiment of the present invention is to carry out the loading process again by way of recirculation of the light function in a part of the plant which is located before the loading, which is advantageous in particular, for example, when these fibers carried in the light fraction are able to achieve a significantly higher level of loading as a result of being loaded once more. The reason may be that, on account of the fiber properties, a longer period for loading or a higher concentration of the chemicals would be more beneficial. In such cases, it is more economical not to tune the process to the “heaviest fibers” but to optimize it economically as a whole.

Another embodiment of the present invention is to use the specific fractions formed differently during the paper or board production. As is known, during the production of paper for the individual grades and, possibly, also layers on the paper machine, different quality requirements are set. It is therefore possible to improve the entire process of paper production by means of the fractionation.

As compared with conventional processes for the production of a fibrous slurry, the loading makes it possible to achieve a higher level of refining with energetic benefits; for example, up to 50% of the refining energy can be saved. This has an effect in particular on all paper grades which pass through a refining process during the production and, above all, in those which have a high or very high level of refining, such as FL (FL fiber loaded) cigarette papers, FL printing and writing paper, FL sack kraft papers and FL filter papers. In these papers, which do not need fillers, free filler which is not deposited in or on the fibers can be removed after the refining process or before the introduction of the fibrous slurry into the machine chest or before the supply to the paper machine. The fibers themselves are, however, provided with filler on the outside and/or inside and outside, so that the positive effects of the fiber loading technology are maintained.

The loading has a positive effect on the production of all paper grades since, as a result of process-induced mechanical loadings in the various sections of the paper machine, such as in the press section, in the drying section or in the region in which the paper web is reeled up, the intermediate product produced and the end product to be produced is highly loaded mechanically by the use of reeling, winding, rewinding and converting machines.

A further advantage in using the technology according to the present invention in the paper grades listed above is that these can also be processed further in a calendar. The fact that, when the fiber loading technology is used, fiber loading particles are deposited in, around and on the fibers means that blackening, i.e. calendar blackening, is avoided.

In one embodiment of the present invention, the starting material used is aqueous fibrous material, in particular aqueous paper stock, from 0.1 to 20% consistency, preferably between 2 and 8%. Calcium hydroxide in aqueous or in dry form or calcium oxide is mixed into the aqueous paper stock in a range between 0.01 and 60% of the proportion of solids present. For the mixing operation, a static mixer, a mixing chest or a pulper system is used; in this case, a pH in the range between 7 and 12, preferably between 9 and 12, is used. The reactivity of the calcium hydroxide is between 0.01 and 10 minutes, preferably between 1 second and 3 minutes. Dilution water is mixed in accordance with predefined reaction parameters.

Carbon dioxide is mixed in accordance with the reaction parameters in a wet paper stock dimension. In the process, calcium carbonate is precipitated in the carbon dioxide atmosphere.

The process temperature preferably lies between −15° C. and 120° C., in particular between 20° and 90° C. Rhombohedric, scalenohedric and spherical crystals are preferably produced, the crystals having dimensions between 0.05 and 5 μm, in particular between 0.3 and 2.5 μm.

In order to produce a fibrous slurry loaded with calcium carbonate, static and/or movable, in particular rotating, mixing elements are used.

The process is preferably carried out in a pressure range between 0 and 15 bar, in particular between 0 and 6 bar. Likewise, the process is advantageously carried out at a pH between 6 and 10, in particular between 6.5 and 9.5. In this case, the reaction time is between 0.03 seconds and 1 minute, in particular between 0.05 and 10 seconds.

It is possible, for example, for precipitation product particles of rhombohedral form with a respective grain size in a range from about 0.05 to about 2 μm to be produced. In specific cases, it is also an advantage to produce precipitation product particles of a scalenohedric form with a respective length in a range from about 0.05 to about 2 μm and a respective diameter in a range from about 0.01 to about 0.05 μm.

According to a preferred refinement of the process according to the invention, the solids concentration of the fibrous slurry provided for precipitation is chosen to lie in a range from about 0.1 to about 60% and preferably in a range from about 15 to 35%.

According to an loading process, expedient practical refinement the carbon dioxide is added of the to the fibrous slurry at a temperature in a range from about 15 to about 120° C. and preferably in a range from about 20 to about 90° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of the present invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a schematic drawing of a plant to illustrate an example of the process according to the invention;

FIG. 2 shows a variant of the process.

Corresponding reference characters indicate corresponding parts throughout the views. The exemplification set out herein illustrates an embodiment of the invention and is not to be construed as limiting in scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and more particularly to FIGS. 1 and 2, there is shown a plant having process steps and apparatuses illustrated by way of example. In this plant, the fibrous material P, for example, chemical pulp or deinked wastepaper, together with water W, is put into pulper 10. Pulper 10 can be constructed conventionally, that is to say contain pulper screen 11 in its bottom region, and also pulper rotor 12, which ensures circulation in the pulper and keeps pulper screen 11 free of blockages. The fibrous material pulped in this way is fed to a number of cleaning apparatuses in the example shown here, which are illustrated symbolically as stock cleaner 14 and stock screen 15. There then follows screw press 16 in order to raise the consistency of the fibrous slurry S again. Press filtrate 18 from screw press 16 can be sent back into pulper 10 for pulping. Thickened fibrous material 19 then passes into crystalliser 20 which, with the aid of carbon dioxide gas 24 that is supplied, is used to trigger the desired precipitation of calcium carbonate in the form of finely distributed filler. Carbon dioxide gas 24 is provided by carbon dioxide supply 21 and can be brought to the desired temperature in heat exchanger 22 with the aid of cooling or heating medium 23. However, there are also other possible ways of controlling temperature, for example, the direct addition of steam, if an increase in the temperature is expedient. The loaded fibrous slurry, possibly diluted with dilution water 27, is collected in storage chest 26.

Then, directly or after intermediate treatment, such as refining, there follows fractionation of the loaded fibrous slurry S′. To this end, by way of example and advantageously, a cleaner plant with highly effective hydrocyclones 13 is used here. These form heavy fraction 2 and light fraction 1. Fibers which are more highly loaded with adhering fillers are specifically heavier than less highly loaded fibers. They can also be stiffer. The fractionation carried out in accordance with the invention, in particular, when sedimentation or centrifugal forces are used, makes use of these differences. It is therefore possible to assume that heavy fraction 2 contains fibers more highly loaded with fillers than light fraction 1.

Here, light fraction 1 is recirculated into the loading process, that is to say led into the feed of screw press 16. Heavy fraction 2 is made available as fibrous slurry 28, for example, to a paper machine, not shown.

The part of the plant shown in FIG. 1 for loading with fillers, in particular calcium carbonate is to be viewed only as a simplified example. In many cases, bleaching can also be integrated. More relating to carrying out the loading can be seen from the publications mentioned at the beginning. The loading process is advantageously operated as an online process in the paper mill.

As already mentioned, the fractionation of the loaded fibrous material can also be used to produce different stock qualities or to provide different qualities during paper production. For this procedure, FIG. 2 shows a schematic drawing of a plant as an example, in which, however, the pulping and the pre-cleaning of the fibrous slurry are no longer illustrated. These can correspond to FIG. 1 or to comparable parts of the plant known per se. With the aid of the process according to the invention, it is possible for example to manage light fraction 1, that is to say that with the less loaded fibers, separately from heavy fraction 2 as far as headbox 32 of a paper or board machine. In this case, the addition of further components 30 and 31 (fibrous materials or additives) normally has to be carried out. It is also possible (not shown here) to process, for example to refine, the fractions differently.

Headbox 32 in this example forms three layers. Light fraction 1 is advantageously led into the inner layer and heavy fraction 2 into the outer layers, in particular, when the higher strength and/or the better retention of the more highly loaded fibers are to be used. A plurality of layers can also be formed with the aid of a plurality of headboxes.

The stock approach system of the paper machine is not shown here. 

1. A process for loading a fibrous slurry with a filler, said filler including at least one of calcium carbonate and calcium hydroxide in one of liquid and dry form, the process comprising the steps of: introducing at least one of said calcium carbonate and said calcium hydroxide into the fibrous slurry; precipitating said filler in the fibrous slurry by chemical reaction; fractionating the fibrous slurry into at least two fractions having a different level of loading; and treating said at least two fractions differently.
 2. The process according to claim 1, wherein said fractionating step further includes the step of applying centrifugal forces.
 3. The process according to claim 2, wherein said fractionating step further includes using hydrocyclones.
 4. The process according to claim 3, wherein said fractionating step further includes the step of producing a centrifugal field with said hydrocyclones, said centrifugal field being at least as great as 200 times the acceleration due to earth gravity.
 5. The process according to claim 1, further comprising the step of forming at least one light fraction having a lower level of loading and at least one heavy fraction having a higher level of loading.
 6. The process according to claim 5, further comprising the step of loading said light fraction again.
 7. The process according to claim 6, further comprising the step of recirculating said light fraction through the same said loading process.
 8. The process according to claim 6, further comprising the step of loading said light fraction in an alternative said loading process.
 9. The process according to claim 5, further comprising the step of refining said light fraction and said heavy fraction.
 10. The process according to claim 5, further comprising the step of producing a plurality of one of paper and board products using one of said light fraction and said heavy fraction.
 11. The process according to claim 5, further comprising the step of producing a plurality of layers in one of paper and board using said light fraction and said heavy fraction.
 12. The process according to claim 1, wherein said fibrous slurry is an aqueous fibrous material, in particular aqueous paper stock, having a consistency between 0.1 to 20%.
 13. The process according to claim 12, wherein said aqueous fibrous material has a consistency between 2 and 8%.
 14. The process according to claim 1, wherein the temperature during said precipitating step is between −15° C. and 120° C.
 15. The process according to claim 14, wherein the temperature during said precipitating step is between 20° C. and 90° C.
 16. The process according to claim 1, further comprising the step of carrying out said precipitating step using gaseous carbon dioxide.
 17. The process according to claim 1, further comprising the step of producing one of rhombohedral, scalenohedral and spherical crystals.
 18. The process according to claim 17, wherein the dimentions of said one of rhombohedral, scalenohedral and spherical crystals are between 0.05 and 5 μm.
 19. The process according to claim 18, wherein the dementions of said one of rhombohedral, scalenohedral and spherical crystals are between 0.3 and 2.5 μm.
 20. The process according to claim 1, further comprising the step of carrying out said loading step at a pressure range of between 0 and 15 bar.
 21. The process according to claim 20, further comprising the step of carrying out said loading step at a pressure range of between 0 and 6 bar.
 22. The process according to claim 1, further comprising the step of carrying out said loading step at a pH between 6 and
 10. 23. The process according to claim 22, further comprising the step of carrying out said loading step at a pH between 6.5 and 9.5.
 24. The process according to claim 1, further comprising the step of carrying out said loading step at a dryness between 0.1% and 60%.
 25. The process according to claim 24, further comprising the step of carrying out said loading step at a dryness between 15% and 35%.
 26. The process according to claim 1, wherein the reaction time of said chemical reaction of said precipitating step is between 0.03 seconds and 1 minute.
 27. The process according to claim 26, wherein the reaction time of said chemical reaction of said precipitating step is between 0.05 and 10 seconds. 